We propose to continue studies in inbred mice to examine the nature of CTLs and CTL precursors which mediate inhibition of the growth of MuLV-induced syngeneic tumors and the availability of effector cell precursors in mice with progressively growing tumors. In addition, the influence of T cells with specificity for a normal T-cell differentiation alloantigen, Thy-1.2, on the growth of tumor bearing this antigen in congenic Thy-1.1 mice will be evaluated. Additional experiments will explore the role of host T cells in modifying the in vivo antitumor activity of passively transferred immune T cells, and the role of purified helper and suppressor T cells and macrophages in regulating the in vitro induction of antitumor CTL responses. The role of each cell type will be defined first in normal and/or immune mice and then examined in mice with progressively growing tumors. These studies should clarify our understanding of host cellular immune mechanisms which regulate antitumor effector responses; they may provide a rational basis for inducing antitumor effector activity by otherwise unresponsive cells from mice with growing tumors.